NAMBA Propwash 04-2023 Web.pub

PROPWASH 10 April 2023 This must be done after a day’s racing to prevent corrosion. Wedging the receiver and battery in place to isolate them from vibration is also a good idea. You can check the water tightness of your radio box by submerging it and blowing down the anten- na tube. Most of the time this will show that the taped cover area is where the leaks are. Of course, 2.4 MHz receiver radio boxes don’t need antenna tubes except in carbon hulls. What happens if something fails? Common fail safes cause the receiver to set the servos to a predetermined position if there’s no signal. The usual choices are throttle off and steering to either right or left turn. None of this works if there’s a power failure in the radio box or a servo failure. A somewhat drastic solution is to have two complete radio systems running from the same transmitter. Each system has a battery, switch, and receiver connected to its own servo. That way if one system fails either the throttle or the steering can be controlled. With this setup it’s important to test both systems independently by switching off each side, one at a time. Since servos usually fail one at a time, some control will be maintained with a single operating servo. All assembled systems need to be tested before running. These days an antenna off range check isn’t needed (or possible with 2.4 radios). The throttle and rudder response needs to be tested before launch. The engine should be running for the rudder check to confirm that ignition interference or vibration isn’t causing a problem. Throttle servos don’t need either power or precision. Analog servos will work well. These days even standard size servos have more than enough power for rudders. Most rudder servos with metal gears and over 100 oz/in torque will be fine. Digital servos give precision response, but ultra-speed isn’t needed. Since ruggedness is important, I wouldn’t use smaller than standard size servos. Outboards are a special consideration. Here the issue is the impact on the servo when the boat flips. In the past, ¼ scale servos were needed, both for torque and for gear size. Today they are probably overkill but still not a bad idea for 7.5 size and larger boats. In any case, don’t use metal servo arms on outboards. See the above radio box picture. The plastic arm will act as a fuse. The splines will strip on overloads. Fixing the problem involves replacing the servo arm, not the servo. Gas outboards need an even heavier duty servo. The servo gearboxes sold by Servo City are often used. Battery safety is the other issue. Today lithium-ion batteries are everywhere. There are spectacular stories about their fires. Despite this, most people feel comfortable carrying around one in their pocket in their cell phones. Why would this be? High quality lithium polymer batteries can be safe when treated carefully. However, precautions need to be taken, especially when charging. Always supervise the charging process and keep the batteries away from any flammable materials. A lipo sack is also a good idea, but the charging process should still be monitored. A charger designed for lipos should be used and the balancing lead must be connected. Follow the manufacturer’s charge current recommendations, but anything over 1C should be supervised carefully. Higher charge rates are possible, especially with the best packs, but magnify battery problems. Your speed control should have a low voltage cut off. Typically, that point is 2.8 volts per cell times the number of cells. However, a bad cell can still overheat when the other cells maintain the battery voltage above the cutoff voltage. The more cells a battery has, the more likely this becomes. Many of the fires at competition events have been in high voltage (over 4S) batteries. A charger that shows the individual cell voltages during charging will reveal a problem cell. A long time for the pack to balance at the end of charging is also a clue to a bad cell. There is no substitute for care and knowing your battery packs. Too high a current draw and over discharging can also cause heating that leads to a fire. A high-quality ESC will often have high temperature and current cut offs. Be careful when testing a boat for the first time. Use a small propeller and check the battery temperature after running. A data logger that records current is very helpful if you are trying to get the last bit of performance, but battery temperature is the important measurement. Temperatures over 140 degrees are a warning to monitor the battery for continuing heat buildup after a run. Normally your batteries shouldn’t get much hotter than 120 degrees. If a battery is hot and/or puffed after a run, it should be placed in a safe area and watched. A metal bucket filled with sand is an example. If a fire starts, don’t panic. This is not some exotic metal fire. Most of what is burning is plastic wrapping and ethylene or polypropylene from the electrolyte. Cooling the battery with lots of water is the solution. Fill the container with water or put the battery in the pond until the battery is cold. In any case get all flammable material away from the fire. The vapor and smoke from any burning plastic is toxic. Ballooned Receiver in a Radio Box Battery Charging